A metal pipeline or pipe installation that is buried underground or under a seabed reacts similarly as a metal immersed in an electrolyte. With changes in the underground environment, such as changes due to rain and humidity, electrical potentials can vary between points along the length of the pipeline to create anodic and cathodic regions. These regions may then form the basis for a corrosion cell in which a cathode (i.e., a point along the pipeline in the cathodic region), an anode (i.e., a point along the pipeline in the anodic region), and an electrical connection between the cathode and anode is formed. When this occurs, the metal within the pipeline that forms the anodic region will be corroded. However, corrosion may be prevented if the electrical connection between the cathode and anode is interrupted. Thus, an electrical insulation gasket may be used between two flanged joints within the metal pipeline to interrupt the electrical current between the cathode and anode.
Both metallic and non-metallic electrical insulation gaskets are used within a variety of industries, including those that process liquids, gases, and gaseous hydrocarbons, to seal flanged joints.
Metallic gaskets can be divided into two principal categories: flat gaskets and ring joint gaskets. Both gaskets are manufactured from a metal or alloy with or without a soft filler or facing. Non-metallic gasket can be manufactured from several materials, each one to meet the demand for each application. The most ordinary is the compressed non-asbestos gasket sheets, manufactured by vulcanization under pressure of mineral or synthetic fibers with a combination of elastomers. Another non-metallic gasket option for high chemical resistance is a restructured polytetrafluoroethylene (“PTFE”) gasket, which can be virgin or blended with other materials. Another conventional embodiment is a phenolic or epoxy resin laminate faced with an elastomer such as styrene-butadiene or styrene-butadiene rubber (“SBR”), nitrile rubber (“NBR”), or chloroprene rubber (“CR”). While non-metallic gaskets adequately provide electrical insulation and fire safe protection, they generally do not provide the mechanical strength necessary for high-pressure applications in pipeline or offshore oil extraction. For these high-pressure applications, a metallic gasket is required to avoid blowout or other failure.
Conventional metallic gaskets that provide fire safe and blowout protection for high-pressure applications are composed of a metal core with two facing materials. One of the facing materials provides the sealing capabilities and the other provides the fire safe protection, while the metal core provides protection against blowout. Metallic gaskets generally form a portion of a sealing assembly and are tested for specific sealing and electrical properties while forming a portion of the sealing assembly. Typical minimum sealability value is of Class BH per ISO standard 15848-1, which corresponds with a sealability of ≦10−4 mg·s−1·m−1 with He at 40 bar and bolts of the sealing assembly tightened to 50% of the yield stress of the bolt material. The required electrical insulation properties are resistivity of 10 mega-ohm (MΩ) with 1000V DC and a dielectric strength of 3 kilo-volts (kV) for 1 minute. Generally, these conventional metallic gaskets have a low flashover voltage threshold and low electrical resistance due to an electrical “leak path” formed near an interface of the two facing materials that provide the sealing capabilities and the fire safe protection.